The invention concerns an active window antenna for motor vehicles having an antenna conductor arranged in or on a windshield and a pre-amplifier circuit with a high-ohmic transistor input in the AM transmission band.
Motor vehicle window antennas and/or radio receivers are described in U.S. Pat. Nos. 3,576,576; 3,587,017; 3,623,108; 3,693,096; 3,771,159; 3,810,184; 3,939,423; 3,965,426; and 3,971,030 and my co-pending U.S. application Ser. No. 829,779, now U.S. Pat. No. 4,163,195 all of which are incorporated herein by reference. As shown, for example, in FIG. 11 of the U.S. Pat. No. 3,771,159 the typical antenna-receiver system of the prior art comprises an antenna mounted in or on the window of a motor vehicle, a preamplifier which is located on or near the window, a feeder such as a coaxial cable, and a radiao receiver. The combination of a window antenna and a pre-amplifier located on or near the window will be referred to as an active antenna.
When transmitter field intensities are too high, the wideband amplifier in the radio receiver can be overmodulated. The critical case of overmodulation appears when a weak transmitter is being received in the vicinity of a strong transmitter. The most unfavorable situation involves a VHF station in the case of the transmitter being received and a long-wave station in the case of the interfering transmitter. In this case, even if amplification control were provided within the radio receiver, it would have no effect because the interfering transmitter lies on a frequency outside the range of the bandwidth of the receiver.
In the case of active rod antennas, overmodulation of the wideband amplifier of the radio receiver may be prevented by bending the antenna rod or, in the case of a telescopic antenna rod, by shortening it.
While overmodulation may be compensated by this means in an active rod antenna, this is not possible in the case of an active window antenna since the position and the length of the antenna conductor are fixed. An object of the present invention is to provide overmodulation of the receiver in an active window antenna.